Probing Johnson Noise and Ballistic Transport in Normal Metals with a Single-Spin Qubit

Thermally induced electrical currents, known as Johnson noise, cause fluctuating electric and magnetic fields in proximity to a conductor. These fluctuations are intrinsically related to the conductivity of the metal. Professors Hongkun Park and Mikhail Lukin and colleagues from Harvard and University of Maryland used single spin qubits associated with nitrogen-vacancy centers in diamond to probe Johnson noise in the vicinity of conductive silver films. Measurements of polycrystalline silver films over a range of distances (20-200 nm) and temperatures (10-300 K) are consistent with the classically expected behavior of the magnetic fluctuations. However, the researchers found find that Johnson noise is dramatically suppressed next to single-crystal films, indicative of a substantial deviation from Ohm’s law at length scales below the electron mean free path. These results are consistent with a generalized model that accounts for the ballistic motion of electrons in the metal, indicating that under the appropriate conditions nearby electrodes may be used for controlling nanoscale optoelectronic, atomic and solid-state quantum systems.